Field of the Invention
The invention relates to an emitter arrangement.
An emitter arrangement of this kind contains at least one thermal electron emitter, which is arranged in a cathode, lies at high voltage and is used to generate thermal electrons (so-called “emission current”). The electrons generated by the emitter are then accelerated in an electric field toward an anode and generate X-rays in the anode material.
The lifetime of a thermal electron emitter in an X-ray tube (surface emitter, filament emitter) is primarily determined by the thermally induced evaporation of the emitter material used, as a rule tungsten. Hence, higher lifetimes can be achieved by either a higher emitter material thickness and/or by a reduced maximum emitter temperature. Here, increasing the material thickness of the emitter achieves a linear increase in the lifetime, while the influence of temperature on the material evaporation is subject to exponential dependence.
Due to its exponential temperature dependence, evaporation results in selective reduction in the layer thickness of the emitter at the hottest point at the start of the lifetime and finally results in emitter burnout. The publication titled “The Lifetime of Incandescent Lamps: The Burnout Mechanism of a Tungsten Wire in Vacuum” [Zur Lebensdauer von Glühlampen: Der Durchbrennmechanismus eines Wolframdrahtes im Vakuum] (in German), by H. Hörster et al. in “Our Research in Germany” [Unsere Forschung in Deutschland], Vol II (1972), pages 76 to 80, Philips GmbH (publisher) describes this process in detail using the so-called “spot model”. The reduction in the layer thickness of the emitter material at the hottest point of the emitter intensifies exponentially during its lifetime. The distribution of the emission from the emitter changes in that the electron emission is concentrated in the direction of the hottest point. Overall, although a constant emission current requires a heating current that is reduced over time, the maximum temperature of the emitter with respect to a constant emission current increases and, to be precise, until the emitter burns out. Depending upon the form of the grain boundaries in the emitter material, at the hottest point, the emitter can be burnt out to way below 50% of the original thickness, although the average loss of thickness, with respect to the entire emitter is only approximately 15%.
A method is also known with which a modified mechanical incorporation of the emitter in the cathode achieves a reduction of the thermomechanical stresses that occur in the emitter during operation.
A reduction in the emitter temperature requires an enlargement of the emission surface and hence an enlargement of the emitter. Hence, focusing of the emitted electrons to form an electron beam generally entails greater expenditure.
Increasing the material thickness in the region of the emission surface (thicker surface emitter sheet, larger filament wire diameter) requires higher heating currents and results in higher thermal inertia. In the case of surface emitters with connecting legs (surface emitters that are not directly welded), it is only possible to bend the connections as far as a certain emitter thickness. This places limits on increases to material thicknesses.
German patent DE 27 27 907 C2 describes a surface emitter with a rectangular emitter surface. The emitter surface has a layer thickness of approximately 0.05 mm to approximately 0.25 mm and is made, for example of tungsten, tantalum or rhenium. Potassium doping with tungsten is also known. The surface emitter produced by rolling has incisions produced by wire erosion or laser cutting methods and arranged in alternation from two opposite sides and transversal to the longitudinal direction. During the operation of the X-ray tube, heating voltage is applied to the cathode's surface emitter, wherein heating currents of approximately 5 A to approximately 20 A flow and electrons are emitted and accelerated in the direction of an anode. When the electrons arrive at the anode, X-rays are generated in the surface of the anode.
In the surface emitter according to German patent DE 27 27 907 C2, the shape, length and arrangement of the lateral incisions enable special types of temperature distribution to be achieved since the heating of a part heated by the passage of current depends on the distribution of the electric resistance over the current paths. Hence, less heat is generated at points at which the electrically active sheet cross section of the surface emitter than at points with a smaller cross section (points with a higher electrical resistance).
The surface emitter disclosed in German patent DE 199 14 739 C1 is made of rolled tungsten sheet and has a circular emitter surface. The emitter surface is divided into conductive tracks extending in a spiral direction spaced apart from one another by serpentine-shaped incisions.
Furthermore, German patent application DE 10 2014 211 688.0 discloses a monolithic surface emitter. Selectively increasing the thickness of the emitter surface at temperature-critical points causes a local temperature reduction at these points (“three-dimensional” emitter concept).
German patent DE 10 2009 005 454 B4, corresponding to U.S. Pat. No. 8,227,970, discloses an indirectly heated surface emitter. The surface emitter contains a primary emitter and a heating emitter spaced apart therefrom both having a circular primary surface. The primary emitter contains an unstructured primary emission surface, i.e. a homogeneous emission surface without slots. The directly heated heating emitter contains a structured heat emission surface, i.e. an emission surface with slots or serpentine-shaped tracks. The primary emission surface and the heat emission surface are substantially aligned parallel to one another and insulated from one another. Further indirectly heated surface emitters are known from published, non-prosecuted German patent application DE 10 2010 060 484 A1 (corresponding to U.S. Pat. No. 8,477,908) and U.S. Pat. No. 8,000,449 B2.
A cathode with a filament emitter (coiled filament) is for example described in non-prosecuted German patent application DE 199 55 845 A1.
Known from published, European patent application EP 0 235 619 A1 is a surface emitter designated a “band emitter” made up of at least two different layers. The surface emitter is, for example, made up of one layer made of tungsten and at least one further layer (for example the two outer layers) made of tantalum. The layers of the surface emitter can consist of different structures of the same material (for example normally structured tungsten and polycrystalline tungsten). This ensures that the grain boundaries at the transition between the two layers are not continued so that the risk of fracture for the surface emitter along such grain boundaries that have developed over the entire material thickness is significantly reduced. However, this does not achieve any reduction in the evaporation of emitter material.
A further alternative, which is based on the field emission of electrons and is therefore known as “field emitter”, is, for example, described in the German patent application DE 10 2014 226 048.5. To date, such field emitters are not used in high-power X-ray tubes.